1. Field of the Invention
The present invention relates to a hydraulic vane pump, and more particularly to the cam configuration (cam curve) to an inner peripheral surface of a cam ring of a hydraulic vane pump such as a balanced-force-type vane pump or an unbalanced-force-type vane pump.
2. Description of the Prior Art
Prior hydraulic vane pumps are subjected to variations in an instantaneous flow discharged therefrom as the rotor rotates, the flow variations being dependent on the configuration of an inner peripheral surface of a cam ring, the position and shape of inlet and outlet ports, the number of vanes used, the compressibility of a fluid being pumped, and other additional configurations. In general, with a vane pump having n vanes, the instantaneous flow undergoes n variations during one revolution of the rotor, and such variations tend to load various parts in a hydraulic system disposed downstream of the outlet port, resulting in pressure pulsations. More specifically, the pressure pulsations are caused in a discharge line by pulsating instantaneous flows discharged by the pump at periods corresponds to intervals between the vanes, and have been a major source of noises and vibrations. Conventional hydraulic vane pumps have been impossible to use in places subjected to strict environmental requirements with respect to noise and vibration. For example, a power steering pump used on an automobile has a severe standard of allowable noise or vibration, and is employed in an environment in which there are noise- or vibration-inducing conditions such as very small air bubbles contained in a fluid pumped by the power steering pump. There have heretofore been available no hydraulic vane pumps for use in such a severe environment which can meet the requirements of use.
Known hydraulic systems incorporating the conventional vane pump have had no basic arrangement for preventing noises and vibrations. A prior attempt for preventing noises and vibrations has been to reduce the volume of a vane chamber while it is in a closed state over a cam curve section corresponding to a maximum lift period or a larger-arc section before the vane chamber is opened into the outlet port, thereby compressing the fluid in the vane chamber so that the fluid pressure in the vane chamber will approach a discharge pressure. This process is known as a pre-compression method. Another attempt has been to use a resilient hose as the discharge line. Large pressure pulsations have been prevented from being propagated to a downstream side by increasing the length of the outlet pipe. The pre-compression method has been effective in reducing instantaneous flow pulsations to a small extent under the conditions in which the compressibility of the fluid being pumped is low and constant and the outlet pressure is kept constant. However, this method has proven unsatisfactory and failed to reduce the pulsations in applications where the above conditions are not present. The soft resilient discharge line or the elongated discharge line cannot achieve satisfactory pulsation reduction, and entirely fails to reduce any pulsations in the pump.
In the accompanying drawings, FIG. 1 shows a typical cam configuration of a conventional balanced-force-type vane pump in which the pre-compression method is incorporated, and FIG. 2 illustrates analyses of fluid behaviors with the cam configuration of FIG. 1. More specifically, FIG. 1 is a cam diagram of the prior vane pump. The cam configuration has one period of .pi. (rad), with a rotational angle .theta. (rad) of the rotor being indicated on an axis of abscissa and a lift L (cm) which is a vane lift length being indicated on an axis of ordinate. FIG. 2 shows in its lower side the waveform of an instantaneous flow of a discharged fluid as calculated on the basis of the above prior cam configuration with the fluid having an extremely high bulk modulus of 12,000 Kgf/cm.sup.2 approximating uncompressibility, and in the upper side the waveform of a pulsating pressure of the fluid in a discharge line of arbitrary design.
As shown in FIG. 2, the instantaneous flow discharged from the prior hydraulic vane pump with the pre-compression method incorporated changes in a region B. With the region B in view, the prior hydraulic vane pump has been improved on the basis of a fundamental cam diagram design concept to reduce the pulsation of the instantaneous flow in the region B, so that flow pulsations can be reduced by precompression in the vane chamber in the large-arc section.
Conventional unbalanced-force-type vane pumps include a cam ring having a cam surface composed of an arcuate curve extending about a certain point and having the same radius of curvature. Where the center of curvature of the arc is displaced from the center of the rotor by a distance and the distance is controllable, noises and vibrations have been suppressed primarily by using many vanes which are odd in number. This is because the instantaneous flow pulsations can be reduced by odd vanes fewer in number than even vanes and the more the vanes the smaller the instantaneous flow pulsations. Since it has therefore been difficult to reduce the size of the prior unbalanced-force-type vane pumps, there are limitations on attempts to make the pump less heavy, increase the efficiency of the pump, operate the pump at higher speeds, and handle the fluid under higher pressures. The known vane pump of this type is difficult to use in applications subjected to stricter standards of noise and vibration.
Where the number of vanes used is to be reduced, say from 10 to 8 vanes, apart from the foregoing problem of pressure pulsations, the following drawbacks occur from the standpoint of the prior design standard: First, where the number of vanes were reduced, the volume of a vane chamber defined between adjacent vanes would be increased and the quantity of air bubbles trapped in the fluid in the vane chamber would be increased. If the vane chamber containing the fluid with a higher air bubble mixture ratio were pre-compressed in the large-arc section, no effect of pre-compression could be achieved. In a compression section, the amount of fluid supplied under discharge pressure into the vane chamber would be increased in the process in which the vane chamber is opened into the outlet port, with the result that an instantaneous flow would be greatly reduced in the period corresponding to a vane-to-vane interval. Secondly, if the number of vanes were reduced, the rotational angle of the rotor corresponding to an expansion section would be reduced, and the slant of the cam surface would become steeper in the expansion section, so that the vanes would be lifted at a higher speed. With the vane lifting speed increased, the flow supplied to the bottom of the vane would locally be increased to cause an increase in flow pulsations. Since the radius of curvature of the inner peripheral surface of the cam ring would be reduced, the ability of the vanes to follow the inner peripheral surface of the cam ring would become poorer, resulting in vibrations, noise, and abnormal wear due to localized separation of the vanes from the inner peripheral surface of the cam ring. For the above reasons, the number of vanes employed in vane pumps cannot be reduced to a large extent. From a technical viewpoint, the minimum number of vanes allowed has practically been ten with balanced-force-type vane pumps and seven with unbalanced-force-type vane pumps. It has been impossible to reduce the number of vanes beyond the above limits. Accordingly, the conventional vane pumps suffer from limitations on efforts to make them more lightweight, smaller in size, and less costly, and demands for vane pumps that are more lightweight, smaller in size, and less costly to construct have not been met.